The precision with which samples are analyzed via NMR has progressed to the point where one part in 10.sup.9 is routinely achieved. The limits to attainable precision include inhomogeneities in applied magnetic fields, noise of various sources, environmental instability and simple asymmetry in structural aspects of the NMR probe. In particular, the structural aspects of the spectrometer components defining and occupying the sensitive region of the NMR probe include the finite magnetic susceptibility of the materials forming the RF coil intrinsically defining the sensitive region. This may include conductors, support forms, adhesives, solder and the like. Measures to reduce inhomogeneities from this type of perturbation are discussed by Anderson, et al. in U.S. Pat. No. 3,091,732 where it was sought to provide susceptibility matched materials and bonding agents for securing the desired homogeneity. These materials are required to exhibit an effective gross magnetic susceptibility approximating air in which these components are necessarily immersed. Thus this prior art may be characterized as matching the structural components of the RF probe (particularly, the coil) to the medium filling the sensitive volume. The present invention seeks to match the susceptibility of the medium to the se same structural components.
The attempt to control matching of susceptibility in solid components of the RF probe will largely be effective at fixed temperature and temperature variation will also vary the volume susceptibility of the surrounding fluid, viz. gas or liquid medium.
In the present invention it is recognized that the presence of particular gasses or liquids in the sensitive region can be a source of magnetic perturbation. These gasses include cryogenic vapors issuing from the magnet cryostat and perhaps from a separate cryostat housing the probe, viz., nitrogen and helium. These cryogenic gasses are vented from the cryostat and settle or permeate the environment. One may also have unspecified gasses employed for the high speed spinner commonly utilized in high resolution NMR spectroscopy.